Optoelectronic semiconductor devices, such as solar cells, lasers, photodetectors, optical modulators, light emitting diodes, and the like, represent an important class of semiconductor devices. These optoelectronic semiconductor devices are enabling devices for applications across a broad range of areas, including medicine, optical telecommunications, military, analytical, astronomy, and energy conversion, to name just a few.
Operating efficiency is a key parameter for the suitability of a device in many applications. Operating efficiency is typically dictated by at least one of two factors: (1) material quality and layer structure of the semiconductors used to form the device and (2) the efficiency with which light can be coupled into or out of the device.
Efforts to improve material quality have resulted in remarkable advances in device performance over the past decade due to advances in deposition methods, equipment, and source materials. Further improvement in material quality, however, is becoming increasingly difficult to achieve.
Several advances in the layer structure of optoelectronic semiconductor devices have also been made over the last decade or so. Multiple quantum-wells, buried oxide layers, exotic guard ring structures, and the like have been developed to try to improve device efficiency, among other reasons. Such exotic layer structures can lead to dramatically increased cost and more complicated fabrication, which can make such devices significantly less attractive for many applications.
The ability to couple light into or out of a device often depends upon the characteristics of an anti-reflection layer (a.k.a., AR coating) that is disposed on the surface of the device. A large mismatch normally exists between the refractive indices of the semiconductor and air, which results in high-reflectivity at the interface of the two materials. An anti-reflection layer is used to “soften” the effect of this the refractive index mismatch by interposing a material having a refractive index that is between those of the semiconductor and air.
A conventional anti-reflection layer that is designed for operation at a specific wavelength has a thickness equal to one-quarter of that wavelength. For devices that operate over a narrow band of wavelengths, such as a laser, a conventional anti-reflection layer can be highly effective. Unfortunately, many optical semiconductor devices, such as solar cells, photodetectors, light-emitting diodes, etc., operate over a range of wavelengths (i.e., a spectral range of interest) that can be as large as hundreds of nanometers (nm). Typically, an anti-reflection layer for such devices is tuned to the center wavelength (as measured in the material comprising the layer) of the required wavelength range. The anti-reflection layer is well-suited for light at this center wavelength; however, its effectiveness rapidly decreases for wavelengths further away from the center wavelength. In order to improve the broad-band performance of an anti-reflection layer in such applications, complicated multi-layer coatings and graded-index coatings are often used; however, such coatings can dramatically increase the cost and complexity of an optoelectronic semiconductor device.